Purpose: Adherence and biologic response to a diet low in total fat (<20% kcal) and high in fruit, vegetables, whole grains and fiber (LFHF Diet) will be measured in healthy, free-living premenopausal women. Dietary assessment and biochemical measures of total fatty acids, antioxidants and phytoestrogens will be used to quantify adherence and biologic response within and between groups. The supplemental effect of soy on these parameters beyond those observed with the LFHF diet alone and added to the usual diet will further be evaluated. Hormonal response relevant to potential breast cancer risk will be compared between groups over time. Background/Significance: The primary hypothesis is that adherence to a dietary pattern recommended for reduced risk of breast cancer is feasible and quantifiable via subjective and objective/biologic measures. Documentation of dietary adherence is essential before valid interpretation of metabolic responses can be formulated. A second hypothesis addresses the growing evidence regarding a potential protective benefit from rich in soy isoflavones, genistein and daidzen. Previous studies of dietary intervention for prevention of breast cancer have concentrated on postmenopausal women. Studies among younger women are few and have small sample sizes, non-generalizable controlled conditions, lack of data timed to menstrual cycles, limited dietary assessment and/or focus on single-macro- or micronutrient without considering the total dietary pattern. By quantifying adherence through both subjective methods and biologic measures, further investigation of hormonal responses to diet and implications for breast cancer risk can be achieved with greater confidence. Through a randomized, placebo controlled trial of LFHF diet with and without soy supplement, the biologic responses in a large group of premenopausal women can be measured sequentially and long term. Methods: 220 premenopausal women (aged 30-45, non-obese, not following low fat diet, regular periods) will be randomly assigned to either usual diet or LFHF diet and will provide dietary, biochemical and hormonal follow-up at baseline (B), 4 (4 mos) and 12 months (12 mos). At 12 mos, both Intervention Group (IG) and Usual Diet Group (UG) will be randomly assigned a soy supplement (40 mg genistein) or placebo within their initial dietary assignment for an additional four months or a total of 16 months. Primary outcome measures include three 24-hour recalls (24 HR), anthropometric measures (ANTHRO), urinary phytoestrogens, serum total fatty acids and plasma carotenoids collected at B, 4 mos, 12 mos and 16 mos as well as menstrual diaries and blood samples for hormone analyses (please see detailed proposal on hormonal responses in this IRPG). Additional blood and urine samples will be banked for further analyses of micronutrients, lipid-lipoproteins and other responses that may accompany adherence to the LFHF-soy enhanced diet especially related to menstrual cycle duration.